The present invention relates to a synchronous motor having a rotor, which is rotatably mounted within a stator having AC (alternating current) windings wound therearound, and a rotation control method thereof, as well as, an inverter for use of such the synchronous motor, and in particular, it relates to a synchronous motor and a rotation control method, in which plural numbers of teeth portions are formed, not only upon a surfaces of each of poles building up said stator, but also upon a surface of said rotor opposing to those poles, respectively, and thereby enabling to obtain a desired characteristic through applying AC voltage thereto from an inverter, as well as, the inverter for use in such the synchronous motor.
Conventionally, when driving a pump and/or a fan, rotationally, as a load thereon, as was already known in the following Patent Document 1, etc., for example, a cryopump driving apparatus is widely used, which is built up with a driving apparatus having a synchronous motor therein, for rotationally driving the load, such as, a refrigerator of a cryopump, etc. And, with such the conventional art, for driving the synchronous motor mentioned above with using a commercial electric power source of frequency 50 Hz, it was already known to conduct the frequency conversion on the frequency of the electric power with using a frequency converter (i.e., an inverter), thereby to drive the synchronous motor with a desired frequency.
On the other hand, as a synchronous motor for general uses or purposes, i.e., being a kind of an AC electric machine, as well as, a rotor having windings wound thereround or that being made of permanent magnet, in the structure thereof, there was already known a synchronous motor, of so-called a type of multi-pole structure, adopting a rotor of applying a permanent magnet(s) in a part thereof, upon the surface of which are formed plural numbers of gutters; i.e., teeth portions, in a direction of the rotating axis thereof. Though such the structure of the rotor, having the teeth portions formed thereround, was also applied, widely, in such as, so-called a stepping motor; however, due to discovery of magnets of the rare-earth elements (for example, neodymium, samarium, cobalt, etc.) having high antimagnetic force, in recent years, it is possible for the motor to obtain a higher torque therefrom, in spite of the same structure or sizes thereof, and also to manufacture it with relatively a large amount thereof, but cheaply; therefore, being widely applied therein.
In addition thereto, for an induction electric machine or motor, there are also known various apparatuses, such as, a voltage controller apparatus for an inverter, in which the inverter built up with a transistor bridge or the like is controlled, so as to obtain an improvement on the characteristics thereof when the motor rotates at a low speed, for example, in the Patent Document 2 which will be listed up below, etc. Namely, with such the conventional technology as was mentioned above, within the control method of maintaining V/F ratio (=φ) constant for satisfying a rated load, in general, and in particular for dissolving shortage of the torque generated from the motor when it rotates at a low speed, the V/F ratio is increased when a frequency is lowered, normally, and at the same time, adjustment is made on terminal voltage of the induction motor in relation with torque of the load, fitting to the characteristic of the load, thereby providing a voltage controller apparatus for the inverter fitting to the load characteristics.
Patent Document 1: Japanese Utility-Model Publication No. Hei 1-14775 (1989); and
Patent Document 2: Japanese Patent Laying-Open No. Sho 57-180395 (1982).
However, in a case when controlling the voltage to be supplied into the induction motor and/or the general-use synchronous motor with using a normal inverter, as was mentioned in the above, however in general, the control method of V/F ratio (=φ) constant is mainly used for the purpose of satisfying the rated load. However, in particular, as was mentioned above, in the case when controlling AC supply voltages for driving the synchronous motor having such the structure, i.e., while forming the plural numbers of teeth portions on the surfaces of the stator poles in the direction of rotation axis thereof, adopting the rotor made of using the permanent magnet in a part thereof, which also forms the plural numbers of teeth portions on the surface thereof in the direction of rotation axis thereof, it is not always possible to obtain sufficient characteristics or performances therefrom. However, this can be considered resulting from the fact that, in particular, the above-mentioned voltage control of applying the inverter therein is mainly achieved for controlling the supply voltages to the induction motor and the synchronous motor, and therefore, it is not always fitting to such the structure of the motor mentioned above, too.